Pressurised systems in the form of compressed air, gas, steam and vacuum are widely used as energy sources in mining and industrial plants. They are associated with a high cost of production, both in energy consumption and financial terms. They also share a common problem: a tendency for leaks.
Inefficiency of compressed air
Compressors are significant consumers of electricity. Compressed air accounts for about 10% of the total electrical power consumed by industry and is recognised as being the highest cost energy source. The primary reason is the heat of compression. Between 80% and 93% of input energy is lost in the form of low grade heat. To produce one unit of output energy in compressed air, at least five times as much energy is consumed in the compression process alone. This is before leaks, system losses and air tool losses. An average compressed air system in South African industry will have a net efficiency of 1%. A large but poorly managed installation has a net efficiency of 0,3%. Compressed air is a precious commodity because of the very low net energy yield.
Energy costs
Three factors contribute to the energy losses: compression, leakage and end-use application. Typically compression and end-use applications require a significant
investment to bring about savings, but they can be substantial. On the other hand leak management is quick to implement and low cost, but can bring about equally substantial savings, with benefits to the business bottom line realised within weeks. A 30% saving on the total cost of compressed air production is a realistic expectation in most situations.
Where and why leaks occur
Leaks fall into two categories: wear and tear related and people related. Wear and tear is a natural consequence of the operation and ageing of a plant and is unavoidable. On the other hand, leaks ascribed to people can be prevented.
Leaks due to wear and tear are dynamic in nature. They will occur with unpredictable frequency with the passage of time. They cannot be prevented but must be searched for, found and fixed at regular intervals. Common sources are:
* Leaks through the walls of damaged or deteriorated hose.
* Loose, defective, missing, makeshift or worn clamps.
* Worn couplings and disconnects.
* Worn cylinder packing.
* Worn or aged valve gland packing.
* Worn O-rings in solenoid and control valves.
* Faulty pressure reducing valves.
* Aged or damaged diaphragms,gaskets and seals.
* Loose flange bolts and couplings.
* Corroded pipes and pipe fittings.
* Cracks, slits and gashes in fixed and flexible pipes and hoses.
* Compression fittings that are loose.
* Worn or contaminated seals on traps, shut off valves and drains.
* Open condensate traps.
* Improperly sealed threads.
People related leaks can be remedied with awareness training and a responsible attitude.
Finding and fixing leaks
In facilities with no leak management in place, losses typically vary between 25% and 50% in industry and 40% and 70% in mines. It is not possible to have no leaks. A realistic target is a leak rate between 5% and 10%.
Finding leaks
There are several approaches to finding the leaks. Most big leaks can be heard but some will escape detection in noisy environments and inaccessible parts of the plant. Smaller leaks are not audible. Most noise produced by compressed air leaks is in the ultrasonic spectrum. By using ultrasonic detectors that filter out audible sound, but are sensitive to the ultrasound produced by the leaks, it is simple to survey an operating production plant and to detect leaks, even in extremely noisy environments.
With directionally sensitive ultrasonic detectors it is possible to pinpoint leak location with great accuracy. With a parabolic dish sensor, which both amplifies the sound and picks up direction, detection and location can be accomplished over substantial distances. This means that large plants can be surveyed quickly and efficiently and inaccessible locations can be inspected at a distance with ease and safety.
Ultrasonic detectors are robust, straightforward to use and relatively low cost, but an outsourced detection service can be a preferred solution.
Quantifying the cost of leaks
Putting a value to the cost of leaks requires knowledge of the magnitude of all the leaks in the system, whether as a percentage of the total compressed air output or in quantified units of air lost. There are various ways of obtaining this information. The most suitable method will depend on the equipment available and the constraints of the production environment.
When a production shutdown is possible, there are three common options: direct measurement, load/unload time and system pressure drop rate. The best option depends on the instrumentation available on the system.
When a production shutdown is not possible, a comprehensive air leak survey can be conducted on a small but representative part of the plant using an ultrasonic detector designed for leak quantification. The leak rates of the individual leaks are added up to arrive at an estimate of the combined leakage, then scaled up for the complete system.
The cost of the leaks is in direct proportion to the system leak rate, expressed as a percentage of the system capacity and the compressed air system lifecycle costing. This figure will be about 5% in a well-maintained system. Poorly maintained systems can be expected to have losses in excess of 40%. One large mining system has been measured at 70%.
In the absence of lifecycle costing, basic estimates can be made using nameplate data. Accuracy can be improved by combining electrical measurements. A leak through a 3,2 mm diameter hole in a typical 700 kPa system requires more than 3 kW of compressor input power. At an electricity cost of R0,30/kWh, the total cost is about R12 500 per year.
The cost to find and fix leaks is small in relation to the cost of the leak. The payback without a formal leak prevention programme is usually a few weeks. A long term solution for continued savings requires the implementation of an ongoing leak management programme with support from senior management.
Leak management programme
A well formulated programme that is seen to have the support of top management is essential. The US Department of Energy’s sourcebook ‘Improving compressed air system performance’ is a good starting point. This in turn should form part of a wider pressurised system loss control strategy for compressed air, steam, gas and vacuum. This would involve a number of steps:
* Determine the baseline for compressed air usage on the plant.
* Get buy-in through awareness campaigns and training.
* Determine the extent and cost of air leaks. This should be used as a baseline to justify the resources that can cost effectively be allocated to the programme.
* Survey the facility to locate and identify the leaks. For most applications an inexpensive handheld ultrasonic detector which locates leaks and gives an indication of the size and intensity is adequate.
* Document the location, type, size and estimated cost of the leak in line with the facility’s predictive maintenance programme.
* Fix the biggest leaks first to get the biggest savings. This will ensure a good start to the air leak programme.
* Document the repairs and associated cost savings to show the effectiveness of the programme. This may identify equipment that presents a reoccurring problem. When this happens, look at the problem in depth for a root cause and develop a permanent solution.
* Compare before and after results to gauge the effectiveness of the programme. Then tell ‘the world’ about the programme and the results that have been achieved.
Air leaks continue to occur, so the programme must be ongoing. Periodic reviews should be done on the system, and the process repeated as necessary to maintain system efficiency.
Conclusion
Compressed air is the most expensive energy utility in common use. Because of its nature, leaks and misapplications are common. When unmanaged, resultant losses are seldom below 35% and can be twice as much. Leaks can be found and fixed at low cost without affecting production. However additional management resources are needed for continued success, with a structured leak management programme. With this in place the payback in previously unmanaged plants can usually be measured in months. Measurement of cost effectiveness can be embedded in the leak management programme.
In the context of process optimisation in an industrial or mining environment, addressing compressed air is a simple, low cost kick-off project, where a large number of people can contribute in an easy way and the results can be measured, reported and seen to support the business bottom line in a significant way in a short space of time.
Presented at a technical seminar hosted by Mining and Industrial Energy Optimisation (MIEO), a chapter of the Southern African Association for Energy Efficiency (SAEE), together with Eskom’s Integrated Demand Management (IDM) division.
For more information contact Mario Kuisis, Martec, +27 (0)11 326 2708, [email protected],
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